A frame structure of the TDD (time division duplex) mode in an LTE (Long Term Evolution) system is shown in FIG. 1. In this frame structure, a radio frame of 10 ms is divided into two half-frames; each half-frame is divided into 10 time slots, each of which has a length of 0.5 ms, and two time slots form a subframe of 1 ms. A radio frame includes 10 subframes (numbered from 0 to 9), and a radio frame includes 20 time slots (numbered from 0 to 19). As for a normal cyclic prefix (CP) with a length of 5.21 us and 4.69 us, a time slot includes 7 uplink/downlink symbols, each of which has a length of 66.7 us, wherein the length of the cyclic prefix of the first symbol is 5.21 us, and the length of the cyclic prefix of the other six symbols is 4.69 us; as for an extended cyclic prefix with a length of 16.67 us, a time slot includes 6 uplink/downlink symbols. Additionally, in this frame structure, the features of the configuration of the subframe are as follows:
Subframe 0 and subframe 5 are always reserved for downlink transmission;
Supporting 5 ms and 10 ms downlink-to-uplink switch-point periodicity;
Subframe 1 and subframe 6 are special subframes and are used to transmit 3 special time slots: DwPTS (Downlink Pilot Time Slot), GP (Guard Period) and UpPTS (Uplink Pilot Time Slot), wherein,
DwPTS is used for downlink transmission and includes at least 3 OFDM (Orthogonal Frequency Division Multiplexing) symbols;
GP is the guard period and does not transmit any data;
UpPTS is used for uplink transmission and includes at least 2 symbols for transmitting PRACH (Physical Random Access Channel).
In case of 5 ms downlink-to-uplink switch-point periodicity, subframe 2 and subframe 6 are fixedly used for uplink transmission;
In case of 10 ms downlink-to-uplink switch-point periodicity, DwPTS exists in the two half-frames, GP and UpPTS exist in the first half-frame, the DwPTS duration in the second half-frame is 1 ms, subframe 2 is used for uplink transmission, and subframe 7 to subframe 9 are used for downlink transmission;
P-SCH (Primary-Synchronization) is transmitted on the third OFDM symbol of DwPTS; S-SCH (Secondary-Synchronization) is transmitted on the last OFDM symbol of time slot 1 and time slot 11;
The present protocol specifies that in a TDD system, the number of symbols in subframe 1 and subframe 6 for transmitting physical downlink control channel signals is 1 or 2. Herein, the signal carried on the channel is directly called as the channel signal.
A radio frame in a FDD (frequency division duplex) system also includes 2 half-frames, wherein each half-frame includes 5 subframes, and each subframe includes two time slots. However, since it is a frequency division system, there is no downlink-to-uplink switch-point or special subframes such as subframe 1 and subframe 6 in a TDD system.
At present, when there is no physical downlink shared channel in a MBSFN (Multiple Broadcast Single Frequency Network) subframe, there is no physical downlink control channel in this subframe either. When there is a physical downlink shared channel in a MBSFN subframe, the number of symbols in the subframe for transmitting physical downlink control channel signals is 1 or 2.
As for other normal subframes (referring to normal subframes in TDD and FDD systems), when the number of available resource blocks of the downlink bandwidth in a radio frame is less than or equal to k (k=10), the number of symbols for transmitting physical downlink control channel signals is 2, 3, or 4; in other cases, the number of symbols for transmitting physical downlink control channel signals is 1, 2, or 3.
At present, the number of symbols for transmitting physical downlink control channel signals is indicated by PCFICH (Physical Control Format Indicator Channel) in the subframe. PCFICH is transmitted on the first symbol of the subframe, and occupies 4 Resource element groups (REG) on the frequency domain. However, the transmission of PCFICH will decrease the utilization efficiency of the system resources, and reduce available PDCCH resources, thus affecting the performance of the PDCCH.
Additionally, in TDD system, the existing protocol specifies that when the number of available resource blocks of the downlink bandwidth is less than or equal to k (k=10) and the downlink-to-uplink switch-point period is 10 ms, the maximum number of symbols in subframe 6 for transmitting physical downlink control channel signals is 2, while the number of symbols in other normal frames for transmitting physical downlink control channel signals is 2, 3 or 4, in such a case, the cell coverage supported by subframe 6 is lower than that supported by other subframes, thus causing a degradation of performance of the system. When the number of available resource blocks of the downlink bandwidth is greater than k (k=10) and the downlink-to-uplink switch-point period is 10 ms, the number of symbols in subframe 6 for transmitting physical downlink control channel signals is at most 2.